The Iraqi army hit back on Wednesday, arresting about 20 suspects and confiscating numerous weapons in a dawn raid in a nearby farming area, said Lt Col Tarik Muhei.

Authorities in the Shia holy city of Karabala, meanwhile, imposed a six-day driving ban starting on Thursday in a bid to protect pilgrims from a wave of sectarian killing.

Donald Rumsfeld, the US Defence Secretary, hinted on Tuesday that US troop levels may increase slightly in the coming days because of the pilgrimages connected to the holiday of Ashura, which ends on 20 March.

Increased attacks marked the celebration during 2004 and 2005.

Rumsfeld said Gen George Casey, the top US military officer in Iraq, "may decide he wants to bulk up slightly for the pilgrimage." He did not elaborate.